Have you got a robinia nearby (false acacia)? Perhaps one called 'Frisia' with yellow/green leaves? That looks like what's called a 'sucker' off the roots of the robinia. The robinia will be a grafted plant and the rootstock will be sprouting for one reason or another - often after damage/severe pruning of the main tree.

Ah, I have a beautiful dwarf growing about ten foot away. It's not damaged but is looking really healthy this year. I believe you are right and this new thing is growing from root stock but growing a normal Robinia, not like the dwarf twisted main tree. Hope suckers aren't going to become a problem as I love my dwarf Robinia - worth a lot of money too.
Thank you

Lyn

Keep mowing! Won't stop them trying but will stop them succeeding!